A photovoltaic solar panel has an output voltage vs. current paramater characteristic that is a family of curves dependent on the solar energy incident on the panel, and the panel temperature. For a given panel temperature, the open circuit voltage of the panel increases as the level of incident solar energy on the panel increases. As temperature increases, the open circuit voltage decreases. Each curve has a maximum power point, where the slope of the curve has a value approximately equal to minus one. The curves have slopes with absolute values greater and less than unity for currents respectively greater than and less than at the maximum power point.
While several systems have been devised to control a photovoltaic solar panel so it operates at the maximum power point, the prior art systems have either been overly simplistic or relatively complex. For example, in Engelhardt. U.S. Pat. No. 3,222,535, a dummy load is selectively connected in circuit with the output of a photovoltaic solar panel and the load connected to the solar panel. A change in panel output power resulting from the insertion of the dummy load is monitored by utilizing a Hall plate that monitors the panel output voltage and current. If no change in the output power is monitored, an indication is provided that the solar panel is operating at the maximum power point. If insertion of the dummy load causes the output power to increase or decrease, indications are respectively derived that the load current derived from the array is to increase or decrease. This prior art system is disadvantageous because it requires passing the load current through a Hall plate, a device incapable of withstanding substantial amounts of current. In addition, analog multiplication devices, including Hall plates, are generally inaccurate, and, therefore, are likely to cause the load current to be set to an improper value for maximum output current.
In Hartman, U.S. Pat. No. 3,384,806, the maximum power point of a photovoltaic solar cell array or panel is controlled by providing a wattmeter responsive to the output of the array. The output of the wattmeter is differentiated. In response to the derivative of the wattmeter output being zero, an indication is provided that the solar panel is deriving the correct output for maximum load. In response to a positive or negative derivative of the wattmeter output, the load current is increased or decreased. The polarity of the wattmeter output is monitored by a relatively complex modulation and phase detecting apparatus.
Engelhardt, U.S. Pat. No. 3,489,915, discloses three different methods of adjusting the maximum power point of a photovoltaic solar panel. One technique requires one cell of the panel to be a "reference" cell that derives an open circuit voltage. Engelhardt assumes that the maximum power point is a predetermined percentage below the open circuit voltage derived by the referenced solar cell. Such a system is inefficient because it requires at least one cell in the panel to be utilized exclusively for control purposes; it is also believed overly simplistic in many instances because the response from a single cell of a panel may not accurately indicate the response of the entire cell and because the open circuit output voltage may not be a fixed ratio of the maximum power point voltage over the entire illumination and temperature range of the cell. In the second method disclosed in the Engelhardt '915 patent, a Hall plate derives an output voltage proportional to the power output of the array and an array regulator varies the load drawn from the array by a small amount. The phase of the power representing output voltage indicates whether an underload or overload condition exists. If the system is delivering maximum available power, the load variation causes excursions on both sides of the maximum power point whereby the Hall plate output voltage is a full recitified squarewave. This method suffers from the same disadvantages described supra in connection with the Engelhardt '535. In another method disclosed in the Engelhardt '915 patent, an a.c. load is connected across the solar panel output and is adjusted to equal the solar panel internal a.c. impedance. The internal impedance of the array is determined by varying the load periodically, to cause the load current, I.sub.b, and load voltage, V.sub.a, to change by .DELTA.I.sub.a and .DELTA.V.sub.a respectively. The a.c. internal impedance of the cell equals .DELTA.V.sub.a /.DELTA.I.sub.a ; maximum power is supplied by the array to the load when .DELTA.V.sub.a /.DELTA.I.sub.a equals V.sub.a /I.sub.a. A pair of multipliers derive the quantities (I.sub.a .DELTA.V.sub.a) and (V.sub.a .DELTA.I.sub.a) and a feedback loop including a comparator equalizes these quantities. Obviously, relatively complex and expensive circuit components, having a tendency to operate inaccurately, are required for such computations.
A further, relatively complex apparatus for assuring that maximum power is supplied by a photovoltaic solar panel to a load is disclosed by Gruber in a paper entitled "High Efficiency Solar Cell Array Peak Power Tracker and Battery Charger", given at the Power Conditioning Specialists Conference, Greenbelt, Md., April 1970; the article can be found on pages 128-136 of the PCSC, 1970 record. In Gruber's article, the output power of the solar cell is monitored by a wattmeter and differentiated. A switching regulator is connected between the array or panel output and the load. A reactance in the switching regulator is periodically perturbed, resulting in a similar perturbation on the output of the wattmeter. The output of the wattmeter is applied to a differentiator, having an output which is applied to a phase detector, also responsive to the perturbing source for the reactance of the switching regulator. The output of the phase detector controls an integrator, which in turn controls on and off times of the switching regulator. It is apparent that the system disclosed by Gruber is relatively complex.
It is, accordingly, an object of the present invention to provide a new and improved system for and method of monitoring and controlling the d.c. output power derived from a photovoltaic solar panel and the power supplied by the panel to a load.
Another object of the invention is to provide a new and improved method of and apparatus for controlling the d.c. output power derived from a photovoltaic solar panel and the power supplied by the panel to a load, wherein relatively simple circuitry that does not require the use of multipliers or wattmeters is employed.